Lead-acid batteries are the standard energy medium storage for solar power systems. They are reliable, relatively cheap, and very heavy. However, they are the weak link in off-the-grid systems because they often suffer from poor care. Their expected life-span is 2 - 12 years, depending on use and abuse. Living on an island with no ferry access, we take particular exception to their weight. Lifting 120 lb batteries in and out of small boats every few years is something we could live without.
Our hope at the start of this project was to do away with the battery bank entirely, replacing it with hydrogen storage and a load-following fuel cell. We had to give that up. The load-following trick is a difficult one, prone to trouble. Batteries are necessary as a kind of "petty cash box" for electric power, allowing the fuel cell to produce a simple, even current. In the end we installed eight L-16 batteries in two 24 volt banks, wired in parallel. (Each bank consists of four 6 volt L-16's, in series.) We could have gotten away with one bank of four; we installed the second bank as insurance.
The solar panels feed into the batteries first, sending power to the electrolyzers only after the batteries are fully charged. The inverter draws DC power from the batteries to produce 120 volt AC current. The fuel cell produces DC which charges the batteries (by contrast, a standard generator produces AC power that passes directly through the inverter). The battery bank is the central reservoir into which PVs and the fuel cell dump power and out of which the inverter draws power.
Thus all power sources pass through the batteries, and all AC power comes from inverting battery DC. This has a number of advantages. First, it prevents duplication: All solar/battery systems include a good inverter as a matter of course. Since fuel cells naturally produce DC, if a fuel cell behaved like a standard backup generator, producing AC, it would need its own inverter, which would be redundant.
Second, most inverters have an internal transfer switch that shuts off the DC inverting function when AC current from a generator passes through the inverter. If your generator has a big kW rating this is fine. But if you have a 1 kW fuel cell, as our budget required, then your AC through-current would be only 1 kW- too small to run the big loads. With the fuel cell feeding DC to the batteries, the transfer switch is never used, and your maximum AC draw is equal to the inverter's rating (in our case 3600 watts). When you have a heavy load such as a table saw, which would ordinarily drain the batteries, the fuel cell effectively acts as a back-up generator, ensuring the batteries don't drain by continuously re-filling them.
The size of your battery bank depends largely on how much hydrogen storage you have, because batteries and hydrogen basically do the same thing, viz. store energy. An L-16 battery is rated at 350 amp-hours at 6 volts, ie 6 x 350 = 2100 Watt-hours. Effective storage is half of full rating, ie 1 kWh, because discharging more than 50% of a battery's power shortens its life drastically. By comparison, a 500 gallon tank of hydrogen at 200 psi, assuming 40% fuel cell efficiency, stores 29 kWh of electric power. (See Useful Equations and Storage Tank for details of these calculations.)
In all cases a small battery bank for "petty cash" is basically unavoidable. So we still have to haul batteries in and out of the boat.
The Battery Room. The batteries are in a separate space to keep off-gassing hydrogen (by-product of charging) from damaging the electronics in the main room. The room is vented to prevent H2 build-up.
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